As one method for measuring a gas concentration, a technique called the Tunable Diode Laser Absorption Spectroscopy measurement (which is hereinafter abbreviated as the “TDLAS” measurement)
In the TDLAS measurement, a laser beam whose wavelength is modulated at frequency f is irradiated into a measurement cell containing one or more gas species to be analyzed, and the power of the laser beam that has passed through the gas is detected by a photodetector. The gas contains a variety of components, each of which absorbs light at a specific wavelength or wavelengths. Therefore, when the center of modulation wavelength is swept at a frequency sufficiently lower than the modulation frequency f, the laser beam undergoes strong absorption at around the wavelength or wavelengths characteristic of a target component of the gas. This spectral absorption line profile in the laser beam intensity is converted to a change in the amplitude of a harmonic component of the modulation frequency f . The n-fold harmonic component of the modulation frequency f (where n is an integer equal to or greater than two) contained in the output signal of the photodetector is extracted by phase-sensitive detection, and the concentration of the target component in the gas being analyzed is determined from the magnitude of the extracted component.
The TDLAS measurement is a non-contact measurement in which the photodetector and other elements do not come in contact with the gas being analyzed. Such a method has the following advantages: The measurement can be performed without disturbing the field of the gas being analyzed; the response time is extremely short and the concentration can be measured in approximately real time; and a high-sensitivity measurement is possible.